The present invention relates generally to oxygen sensors capable of detecting oxygen. Particularly, the present invention relates to an electrode composition for an oxygen sensor.
Exhaust sensors are used in the automotive industry to sense the composition of exhaust gases such as oxygen, hydrocarbons, and oxides of nitrogen, with oxygen sensors measuring the amounts of oxygen present in exhaust gases relative to a reference gas, such as air. A switch type oxygen sensor, generally, comprises an ionically conductive solid electrolyte material, a sensing electrode which is exposed to the exhaust gas, and a reference electrode which is exposed to the reference gas. It operates in potentiometric mode, where oxygen partial pressure differences between the exhaust gas and reference gas on opposing faces of the electrochemical cell develop an electromotive force, which can be described by the Nernst equation:   E  =            (              RT                  4          ⁢          F                    )        ⁢          ln      ⁡              (                              P                          O              2                        ref                                P                          O              2                                      )            
where:
E=electromotive force
R=universal gas constant
F=Faraday constant
T=absolute temperature of the gas
PO2ref =oxygen partial pressure of the reference gas
PO2=oxygen partial pressure of the exhaust gas
The large oxygen partial pressure difference between rich and lean exhaust gas conditions creates a step-like difference in cell output at the stoichiometric point; the switch-like behavior of the sensor enables engine combustion control about stoichiometry. Stoichiometric exhaust gas, which contains unburned hydrocarbons, carbon monoxide, and oxides of nitrogen, can be converted very efficiently to water, carbon dioxide, and nitrogen by automotive three-way catalysts in automotive catalytic converters. In addition to their value for emissions control, the sensors also provide improved fuel economy and drivability.
Further control of engine combustion can be obtained using amperometric mode exhaust sensors, where oxygen is electrochemically pumped through an electrochemical cell using an applied voltage. A gas diffusion-limiting barrier creates a current limited output, the level of which is proportional to the oxygen content of the exhaust gas. These sensors typically consist of two or more electrochemical cells; one of these cells operates in potentiometric mode and serves as a reference cell, while another operates in amperometric mode and serves as an oxygen-pumping cell. This type of sensor, known as a wide range, lambda, or linear air/fuel ratio sensor, provides information beyond whether the exhaust gas is qualitatively rich or lean; it can quantitatively measure the air/fuel ratio of the exhaust gas.
The solid electrolyte commonly used in exhaust sensors is yttria-stabilized zirconia. This material is an excellent oxygen ion conductor under various exhaust conditions. The electrodes are typically platinum-based and are porous in structure to enable oxygen ion exchange at electrode/electrolyte/gas interfaces. These platinum electrodes may be co-fired or applied to a fired (densified) electrolyte element in a secondary process, such as sputtering, plating, dip coating, etc. Co-fired electrodes are often used in planar type sensor elements, in which the electrodes may reside between laminated layers, where many secondary processes are not accessible. In this case, a thick film paste is may be screen printed onto unfired (green) ceramic tape and dried. The screen-printed tapes are then stacked, laminated, cut, and fired to make sensor elements.
The materials and processes used to fabricate sensor elements often provide a source for contaminant impurities that degrade the performance of the electrochemical cell. These impurities, especially silicon-based impurities, tend to migrate by diffusion, for example in the firing process, creating a barrier to oxygen ion conduction at the electrode/electrolyte interface. There exists a need in the art for an electrode formulation that is more tolerant of material and process impurities, which will help provide a cell with low, stable electrode/electrolyte interfacial impedance.
The deficiencies of the above-discussed prior art are overcome or alleviated by the oxygen sensor, electrode, and methods of the present invention. The sensor comprises: a sensing electrode having a first electrical lead and positioned to sense a sensing gas; a reference electrode having a second electrical lead; and an electrolyte disposed between and in intimate contact with said sensing electrode and said reference electrode; wherein at least one of said sensing electrode and said reference comprises about 80 wt % to about 99.85 wt % noble metal, about 0.1 wt % to about 14 wt % non-alumina metal oxide, and about 0.05 wt % to about 6 wt % alumina, based upon the total weight of the electrode.
The electrode comprises: about 80 wt % to about 99.85 wt % noble metal, about 0.1 wt % to about 14 wt % non-alumina metal oxide, and about 0.05 wt % to about 6 wt % alumina, based upon the total weight of the electrode; wherein the electrode has a resistivity below about 500 milliohms/square at about 25xc2x0 C.
The method of making the electrode comprises: forming an ink comprising about 75 wt % to about 95 wt % platinum; about 0.1 wt % to about 11 wt % non-alumina metal oxide; greater than about 0.1 wt % alumina and up to about 15 wt % fugative material, based upon the total weight of the ink; applying said ink to at least a portion of one side of a substrate; and sintering said substrate to form a first electrode on said substrate; wherein the electrode has a resistivity below about 500 milliohms/square at about 25xc2x0 C.
One of the methods of making the sensor comprises: forming a first ink comprising about 75 wt % to about 95 wt % noble metal; about 0.1 wt % to about 11 wt % non-alumina metal oxide; greater than about 0.1 wt % alumina and up to about 15 wt % fugative material, based upon the total weight of said first ink; applying said first ink to at least a portion of a first side of a first substrate; applying a second ink to at least a portion of a second side of a second substrate; contacting electrical leads to said first ink and said second ink; disposing an electrolyte between and in physical contact with said first side and said second side to form an assembly; forming a protective layer over said first substrate; and sintering said assembly to form the sensor having an electrode having a resistivity below about 500 milliohms/square at about 25xc2x0 C.
Another method of making the sensor comprises: forming a first ink comprising about 75 wt % to about 95 wt % platinum; about 0.1 wt % to about 11 wt % non-alumina metal oxide; greater than about 0.1 wt % alumina and up to about 15 wt % fugative material, based upon the total weight of said first ink; applying said first ink to at least a portion of a first side of an electrolyte; applying a second ink to at least a portion of a second side of said electrolyte to form an assembly; connecting electrical leads to said first ink and to said second ink; forming a protective layer over said first side; and sintering said assembly to form the sensor having an electrode having a resistivity below about 500 milliohms/square at about 25xc2x0 C.
Finally, one method of sensing exhaust gas comprises: using a sensor comprising a sensing electrode having a first electrical lead, a reference electrode having a second electrical lead, and an electrolyte disposed between and in intimate contact with said sensing electrode and said reference electrode; wherein at least one of said sensing electrode and said reference electrode comprises about 80 wt % to about 99.85 wt % noble metal, about 0.1 wt % to about 14 wt % non-alumina metal oxide, and about 0.05 wt % to about 6 wt % alumina, based upon the total weight of the electrode; disposing said sensor in an exhaust stream; and contacting said sensing electrode with exhaust gas.
The above-discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings.